"Color Filtering" at the Atomic Level

One of the most astounding inventions of the late 20th century, the scanning tunneling microscope, or STM, yields atomic-scale landscapes of electrically conducting surfaces such as metals. Now, researchers at the Colorado School of Mines (Peter Sutter) have demonstrated a new technique, called "energy-filtered STM," which is analogous to putting a color filter on an ordinary microscope. Just as color filters make it easier to discern desired features in a photograph, color-filtered STM makes it easier to distinguish between chemically similar atoms, something that's usually very difficult to do. It can even identify specific chemical bonds on a surface. Conventional STMs employ a metal tip, which, as it turns out, is generally most sensitive to the highest-energy electrons on the surface. These electrons jump or "tunnel" to the tip, giving scientists data to reconstruct an image of the surface. This preference for the highest-energy electrons can be a problem, because it can obscure the signal from lower-energy electrons, which may be associated with different atoms or different kinds of chemical bonds. To address this issue, the new technique employs an indium arsenide (InAs) tip. InAs is a semiconductor, and all semiconductors have a "fundamental bandgap," a range of energies that no electrons can possess because of the 3D atomic structure of the material. In the case of a semiconductor tip very close to a conducting surface, what's more important is something called a "projected gap," a range of forbidden energies that appears when the 3D electronic structure is seen along the tip axis. So because of the projected gap, electrons in a certain energy range cannot tunnel to the tip. Adjusting the voltage between the tip and sample can shift this projected gap so that it blocks off the high-energy electrons, making the tip more sensitive to electrons in lower-energy bonds at the sample surface (see images). Researchers can shift this range of forbidden electron energies repeatedly, to build up, for example, maps of specific chemical bonds on a surface, and to analyze how abundant one type of chemical bond is compared to others. This technique is now being explored for 'atom-by-atom' mapping of the composition of alloys of chemically similar elements, which is important for certain technologies such as thin-film growth, which often involve nanometer scale variations in the composition of alloys (Sutter et al., Physical Review Letters, 25 April 2003)